Why would you want a helmet cam to be inconspicuous? I find that, when I have my helmet light on (which people frequently mistake for a camera), drivers give me significantly more room. When drivers think they're on camera, they behave better.
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Neil FeinAug 15 '12 at 23:55

These are both older models compared below. The newer models on the websites linked have better specs, and better resolution.

Edit:

In response to (now deleted) comments below.

The Contour HD is a good camera, with fairly short (sub 2 hour) battery life. It does have the pretty cool geotagging feature as well. I would recommend the GoPro cameras, but it would be best if you bar mount those, and they aren't as small. Not huge, but not tiny.

I bought a DOGCAM Bullet HD Wide camera, on the basis of a decent camera at a good price. The Contour HD is a nice camera but too expensive for me.

The DOGCAM Bullet is compact unit, 80mm long and 22mm diameter. Waterproof so strapped to my helmet for kayaking, on the bike I tend to strap it to the bars. It'll last a couple of hours on a battery charge, it's an internal battery which is charged via the mini USB port in the back of the camera. Plugging in via the USB it just appears as a removable flash drive on the computer to copy the video's.

My only real gripe with it is the on/off switch, there's very little tactile feedback to know if I've pressed the button. It does have a small LED at the front to indicate when it's recording or the memory card is full but in daylight conditions it's not the easiest thing to spot. When it's on my bars I really have to lean forward and squint to see the LED.

For inconspicuousness, it looks a lot like a regular front bike light, if that's an acceptable disguise.
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drfrogsplatAug 16 '12 at 6:10

I use one of these and I agree with everything you say including the gripe. (Although I believe there is an upmarket version which will shine a laser for 10 seconds to let you know its come on.) Mine cost around GBP100 a year ago - I mention this because only a few weeks ago I bought my daughter a cam from Amazon which looks exactly like the dogcam bullet, still offers HD recording, but was half the price. Have yet to see quality...
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PeteHAug 17 '12 at 14:57

I'd wait for Liquid Image's Ego to be released this June. $149, 1080p HD, small, waterproof, rechargeable, built-in wifi so you can monitor and control from your iPhone or Android. Buy a black one, and maybe you can embed it into a bicycle helmet without compromising the integrity too much. This camera was announced at the Consumer Electronics Show this January, 2012.

After I fried a my first pair from water getting in by the buttons and USB port, I put black electrical tape over the buttons and seam, and they are now effectively waterproof. The battery life is 2.5-3 hours, and they record to my 16GB MicroSD card, which can then be put right into my computer. The two buttons are easy to find and press (even under the tape).

If being inconspicuous is the main thing, then you could try a smartphone in one of those plastic pouches that you find in outdoor shops. You can carry it around your neck, or attach it to rucksack straps or similar, and most observers would assume that's just how you carry your phone.

There are various spy camera apps that can be set to take regular pictures or video on a loop. It depends on your phone and the app how easy it is to start the video, but for most purposes it might be enough to start at the beginning of the commute and just record. Downloading/managing the files is easy, and battery life of modern phones is amazing. Quality might not be as good as a dedicated camera, but should be ok for most purposes.

As you mention commuting, I assume your main aim is to record evidence if something goes wrong, and for that the quality of a mobile phone cam should be enough. Many of the other recommendations are action cams that are meant for creating impressive vids of how you handle a difficult mountain bike trail or something, but they may not be best for everyday traffic situations. For example, I have a GoPro Hero which is great (can start video with one big button), but one limitation is that the video resolution and fps rate is so high that an SD card fills up in one or two hours (and it's bulky). High fps is important for action shots, but for traffic offences you only need a much lower fps and therefore smaller file sizes.

In any case, it's worth experimenting with a smartphone app and carrying pouch as you probably carry a phone anyway.

I originally bought a pair of Lorex sunglasses for this purpose but found them to be very uncomfortable. There is a built in light that can be seen out of the side vision so all of the components are already set up for a helmet. Looks easy enough to strip the components out of them and embed them in and behind the lining around the corners of my helmet so that the little pin cam can be looking out through the wind visor above my forehead. Probably not a solution but it is an experience to share.